"While we haven't had any reports of unauthorized activity on Dropbox accounts, we've taken a number of precautionary steps and continue to work around the clock to make sure your information is safe," the post says.

On Monday, users of the forums had begun complaining about receiving a flood of spam in their e-mail. Some of them said the e-mail address impacted is one they have only used to create their Dropbox account.

Many of the users who complained are from Europe and at least some of the spam messages are scams masquerading as messages from European gambling sites.

Many users also had wondered whether an hour-long outage Tuesday was related to the problem. But in the forum post, Dropbox said the outage "was incidental and not caused by any external factor or third party."

It has been just over a year since a glitch briefly let people use any password they wanted to sign into any Dropbox user's account. The company said that problem, which lasted for about four hours, was caused by errors in a code update.

Dropbox has roughly 50 million users who, according to the site, upload a billion files to the service every 48 hours.

The company has emerged as a leader in the emerging field of cloud, or Web-based storage. The cloud is designed to save hard-drive storage space for users by instead letting them store and retrieve documents to multiple devices from remote Web servers.

That kind of remote storage is being viewed as the future of computing by companies like Apple, Google and Amazon, all of whom offer cloud-storage features. But it has raised the obvious concerns among some security analysts, who say remotely stored info could become a tempting target for hackers.